The hyperuricemia causes gout and renal insufficiency and further is considered to be a factor causing coronary disease. Furthermore, the hyperuricemia is suggested to closely relate to development of diseases of adult people such as hypertension. Therefore, treatment of the hyperuricemia can be effective not only for treating gout but also for preventing various diseases relating to daily nutrition and developing in the course of advancement of age.
At the present time, the hyperuricemia is treated using an inhibitor for inhibiting production of uremic acid such as allopurinol and an accelerator for uricotelism such as benzbromalone. However, the allopurinol is well known to cause side effects such as lesion, hepatopathy, and myelogenetic troubles. The allopurinol and its metabolic product (oxypurinol) are excreted from kidney. However, if the excretion of uric acid decreases, the excretion of these compounds also decreases and the their concentrations in blood increase. Therefore, the chance of causing side effects increases.
It is reported that benzbromalone also causes hepatopathy. Accordingly, it is desired to develop new pharmaceuticals so that the practitioners can select most appropriate pharmaceuticals.
Recently, the below-mentioned xanthine oxidase inhibitors having no purine nucleus such as TMX-67 (Teijin Corporation, Patent Publication 1: WO 92/09279), Y-700 (Mitsubishi Wellpharma Corporation, Patent Publication 2: WO 98/18765) and KT651 (Kotobuki Corporation, Patent Publication 3: JP-A-12-1431) have been reported:

The present inventors made studies on bicyclic condensed hetero rings having structure differing from the above-mentioned structures and filed a patent application (Patent Publication 4: WO 03/042185).
The inventors further have made studies and discovered that compounds of the below-mentioned formula (I) which are 4-hydroxy-2-phenylthiazolo[5,4-d]pyrimidine or 4-hydroxy-2-phenyloxazolo[5,4-d]pyrimidine in which its phenyl group has cyano in the 3-position and phenoxy or the like in the 4-position have a xanthine oxidase inhibiting effect. The present invention has been completed based on the discovery.